mpc 500 and chargable batteries

Hi there, i have some 800 mAh Batteries in the MPC and I am finding i fully charge them and the until is only registering 6 v for all the batteries and it dies after 2 hours use and it screeches as the battery dies more, does anyone have any suggestions or any chargeable batteries they use that work for more than this and power the unit up fine?

thanks

Sony! This is what I use:
http://www.amazon.com/Sony-2500-Recharg ... 479&sr=8-1

Fully Charged it Registers 7.32v. Lasts more than 6 hours

I use these, works great

http://www.amazon.com/Duracell-Charged-Rechargeable-Batteries-4-Pack/dp/B000XSA60I/ref=sr_1_1?ie=UTF8&s=electronics&qid=1255919878&sr=8-1